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POTABLE WATERS                                     g

spends with that in the nitrometer tube, and the volume of gas read off,
the temperature and barometric pressure being observed. The bulb C is
then lowered and concentrated caustic potash solution introduced into the
nitrometer by means of the funnel D and the tap d, care being taken to avoid
entry of air. The instrument is then well shaken so that the caustic potash
may absorb the carbon dioxide and the volume of the remaining gas read
off after the two levels of the liquid have been brought into agreement by
raising C. These operations are subsequently repeated after a freshly
prepared concentrated solution of pyrogallol in excess of potassium hydroxide
has been introduced into the nitrometer, the latter being left for about an
hour after shaking in order that all the oxygen may be absorbed. The
volume of the residual gas is then read with the usual precautions.

The first reading gives the total volume of the three gases, C02 + N 4- 0,
the second the volume of N + 0 and hence, by difference, the C02, and
the third the N and therefore, by difference, the 0. The volumes thus
obtained must be reduced to the temperature o and the pressure 760 mm.,
this being done by means of the formula,

V = _JL^^A_,
760  (i + 0-00367 t}'

where F is the volume of gas at o and 760 mm., v the volume actually read
off, P the barometric pressure, p the correction of the pressure for the
temperature t (found from suitable tables), / the correction of the pressure
for the water vapour (from tables), and t the temperature. Finally the
volumes of the different gases are calculated per 100 litres of the water.

2. Carbonic Anhydride.As a rule, estimations are made of the
total carbon dioxide, that in the free state and that semi-combined, i.e.,
existing as bicarbonates.

(a) TOTAL CARBON DIOXIDE. To 0-5 or i litre of the water, collected
so that no trace of gas escapes in a flask which is almost completely filled,1
are added 5-10 c.c. of concentrated ammoniacal calcium chloride solution,2
the flask being then closed tightly with a rubber stopper. After some time
(24 hours or more) the water is heated on a steam-bath, access of air being
avoided as far as possible ; it is then filtered rapidly, the filter being kept
covered with a glass. The precipitate of calcium carbonate is washed with
boiled water until ammonium oxalate fails to render the wash water turbid.
Any calcium carbonate adhering to the walls of the flask is dissolved in a
little dilute hydrochloric acid and the solution and water used for sub-
sequent washing out of the flask placed in a small beaker, where the calcium
carbonate is reprecipitated by addition of hot sodium carbonate solution ;
this precipitate is filtered on the original filter and the whole washed until
the wash water is no longer alkaline.

The filter with the calcium carbonate is introduced into an apparatus
for the determination of carbon dioxide (see chapter on Cement Materials).
The weight of C0a found is then referred to 100 litres of water.

1  It is convenient to use a flask tared with its rubber stopper, and to weigh it after
nearly filling it with the.water;   from the weight of the water the volume may be
obtained by dividing by the density.

2  This operation is best carried out where the water is collected.  .         ,         .         4         _              ~, (.